An ad hoc wireless network is one in which a packet may travel multiple wireless hops to reach its destination. It consists of a set of nodes or stations that autonomously discover each other, self-organize, and establish peer-to-peer communications without the presence of a centralized controller or fixed infrastructure. In the most common form of ad hoc networks, each node is potentially a wireless router of packets originated by itself or other nodes. Ad hoc wireless networks may be mobile or stationary. Other terms commonly used for multi-hop wireless networks include multihop networks, packet radio networks, and mesh networks, the last term is typically used only for stationary networks.
Ad hoc networks require different technology and algorithms from conventional wireless networks, e.g., cellular telephony systems. In cellular systems, the base stations provide network access to the handsets, which only need to go over one wireless “hop,” rather than the multiple hops used in ad hoc networks. Furthermore, the base stations themselves do not move in cellular systems, so they can have very reliable, long-term interconnections such as point-to-point microwave or fiber links. In ad hoc networks, by contrast, there is no fixed infrastructure and, in the most general case, everything is moving.
Data communications in today's Mobile Ad hoc Networks (MANETs) is almost universally architected based on the layering concept. As shown in FIG. 1, at each relay node, a packet goes “up” three layers and then “down” three layers, with a number of processing and queueing steps along the way. Crucially, the packet has to re-contend for access to the shared channel at every single node along the way to the destination. This operational regime fundamentally limits the performance, chiefly the latency, and energy efficiency, of current ad hoc networks. These limitations on performance impact the quality of service provided by ad hoc networks and therefore limits their application as solutions for communication problems. Moreover, this regime creates a bottleneck that prevents technological advances in protocols and waveforms from translating into bottom-line end-to-end performance.
Latency has been a data communication issue for decades. Circuit switched communication networks have grappled with this problem for years, particularly in the context of providing the necessary quality of service needed for voice communications. Latency has also been a problem for packet switched networks. To address packet latency issues, scientists and engineers have developed cut through forwarding techniques that can reduce latency and increase quality. Similarly, wormhole routing techniques have been developed that reduce the processing needed to forward a data packet and therefore reduce latency. However, these techniques for wireline networks do not require the medium access arbitration that is required in a wireless ad hoc network. Moreover, wireline techniques do not need to decode wireless signals during the forwarding process and therefore avoid, at least largely, the latency delays arising from wireless signal processing.
Latency is a particularly severe problem for wireless ad hoc networks, because all the nodes in the network must contend for access to a shared RF medium. To date most wireless ad hoc networks have used a form of channel access protocol similar to that mandated by the IEEE 802.11 standard, in which a transmitting node first sends a Request to Send (RTS) signal, an intended receiving node then replies with a Clear to Send (CTS) signal if the transmission may proceed, and if so, the transmitting node then sends a data frame, followed up by an acknowledgement (ACK) from the receiving node if it was correctly received. This channel access protocol has numerous defects when used in an ad hoc network, e.g., the guard times needed for the transmission propagation may take a significant fraction of the time used for actual communication (or even more), multiple RTS/CTS interactions may need to take place before a data frame can actually be transmitted, and so forth. As a result, the delay for sending a single packet across multiple hops may be both variable and unacceptably high.
These problems exist in both stationary ad hoc networks and mobile ad hoc networks. In stationary ad hoc networks, channel access can be an issue because traffic patterns may change across the network as users transmit files, cease for a few minutes, then inspect web pages, etc. In short, bursty and unpredictable user traffic causes channel access issues for stationary ad hoc networks, which in turn causes unacceptably high delay (latency) for delivering such traffic. The problems are even worse in mobile ad hoc networks, where the hop-by-hop paths through the network change unpredictably even as these bursty traffic flows must be accommodated.
There therefore exists a need in the art for a transport process that provides reduced latency for a wireless ad hoc network.